1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units that are formed by sandwiching an electrode assembly between separators.
2. Description of the Related Art
Among fuel cell units, there is one type that is formed in a plate shape by sandwiching between a pair of separators an electrode assembly that is formed by placing an anode electrode and a cathode electrode respectively on either side of a solid polymer electrolyte membrane. A fuel cell is formed by stacking in the thickness direction of the fuel cell units a plurality of fuel cell units that are structured in this way.
In each fuel cell unit there is provided a flow passage for fuel gas (for example, hydrogen) on the surface of the anode side separator that is positioned facing the anode electrode, and there is provided a flow passage for oxidizing gas (for example, air that contains oxygen) on the surface of the cathode side separator that is positioned facing the cathode electrode. In addition, a flow passage for a cooling medium (for example, pure water) is provided between adjacent separators of adjacent fuel cell units.
When fuel gas is supplied to the electrode reaction surface of the anode electrode, hydrogen is ionized here and moves to the cathode electrode via the solid polymer electrolyte membrane. Electrons generated during this reaction are extracted to an external circuit and used as direct current electrical energy. Because oxidizing gas is supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react to generate water. Because heat is generated when water is created at the electrode reaction surface, the electrode reaction surface is cooled by a cooling medium made to flow between the separators.
The fuel gas, oxidizing gas (generically known as reaction gas), and the cooling medium each need to flow through a separate flow passage. Therefore, sealing technology that keeps each flow passage sealed in a fluid-tight or airtight condition is essential.
Examples of portions that must be sealed are: the peripheries of supply ports that are formed so as to penetrate the separators in the thickness direction thereof in order to supply and distribute reaction gas and cooling medium to each fuel cell unit of the fuel cell; the peripheries of discharge ports that are formed so as to penetrate the separators in the thickness direction thereof in order to collect and discharge the reaction gas and cooling medium that are discharged from each fuel cell unit; the outer peripheries of the electrode assemblies; and the space between the separators of adjacent fuel cell units. Organic rubber that is soft yet also has the appropriate resiliency or the like is employed as the material for the sealing member.
FIG. 35 is a plan view showing a conventional fuel cell stack. In FIG. 35 the reference numeral 4 indicates a communication port such as a fuel gas supply port and discharge port, an oxidizing gas supply port and discharge port, and a cooling medium supply port and discharge port that each penetrate the fuel cell stack 1 in the direction in which separators 2 and 3 are stacked. The reference numeral 5 indicates an area in which a plurality of fuel gas flow passages, oxidizing gas flow passages, and cooling medium flow passages running along the separators 2 and 3 are formed.
FIG. 36 is a longitudinal cross-sectional view of a conventional fuel cell stack 1 taken along the line X—X in FIG. 36. As can be seen in plan view, in order to make the space occupied by the sealing member, that does not contribute to power generation, as small as possible, conventionally, by locating gas sealing members 8 and 9, which respectively seal a fuel gas flow passage 6 and an oxidizing gas flow passage 7, together with a cooling surface sealing member 10, which seals a cooling medium flow passage, aligned in a row in the stacking direction of the fuel cell units 11, the outer dimensions in the stacking direction of the fuel cell stack 1 are minimized.
According to FIG. 36, the fuel gas supply port 4 and the fuel gas flow passage 6 that are isolated in a sealed state by the gas sealing members 8 and 9 are connected by a communication path 12. The communication path 12 is provided so as to detour around, in the thickness direction of the separator 2, the gas sealing member 8 that seals the entire periphery of the fuel gas flow passage 6. More specifically, the communication path 12 is formed in such a way that grooves are formed between the fuel gas supply port 4 and fuel gas flow passage 6 of the separator 2, and a bridge plate 13 is provided over the grooves.
Moreover, the separator 3 also has a similar communication path (not shown) near the oxidizing gas communication port (not shown). Such a structure is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 10-74530.
However, because the bridge plate 13 is a separate element that is installed on the separator 2 so as to be substantially flush with the separator 2, a gap 14 is inevitable between the separator 2 and bridge plate 13 at a connecting portion therebetween, as shown in FIG. 37 (the same description applies in the case of the separator 3). FIG. 37 is a longitudinal cross-sectional view, showing a conventional fuel cell stack 1, taken along the line Y—Y in FIG. 36. The drawback with this structure is that sealing performance may be lost if the gas sealing members 8 and 9 are deformed in the gap 14 when the gas sealing members 8 and 9 are attached onto the separators 2 and 3 and the bridge plate 13. Moreover, another drawback is that it is very difficult to form the gas sealing members 8 and 9 on the separators 2 and 3 and the bridge plate 13, because the material of the gas sealing members 8 and 9 may leak through the gap 14.
As disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-148252 and U.S. Pat. No. 6,066,409, a technique in which through holes are formed in a separator that penetrate in the thickness direction of the separator, and a reaction gas is made to flow into a gas flow passage via the through holes, has been proposed. However, in such a structure, a problem is experienced in that, because reaction gas flow passages must be formed between the adjacent separators not only on the front surface of the separator but also on the rear surface thereof in order to allow the reaction gas to flow from the gas flow passage on the rear surface (cooling surface) to the gas flow passage on the front surface or vice versa, the thickness of the fuel cell units may be increased by the amount necessary to form these flow passages.
Moreover, it is necessary to locate the cooling surface sealing member so as to be offset inward (toward a reaction surface) from the gas sealing member for ensuring a space to form the through paths in order to allow the reaction gas to flow in from the rear surface of the separator because the cooling surface sealing member that seals the cooling medium flow passage is provided on the rear surface of the separator. As a result of locating the cooling surface sealing member so as to be offset from the gas sealing member, the cooling surface sealing member is located at a position overlapping, as viewed in the stacking direction, with the reaction gas flow passage that is located inward from the gas sealing member. In this case, the minimum thickness of the fuel cell unit equals to the sum of the thickness of the reaction gas flow passage and the thickness of the cooling surface sealing member. Moreover, if the through holes are provided not only on the separator adjacent to one electrode but also on the separator adjacent to the other electrode, the minimum thickness of the fuel cell is doubled. If a fuel cell stack is formed by stacking a plurality of such fuel cell units, the overall thickness of the fuel cell stack is found by multiplying the number of stacks by the minimum thickness of each fuel cell unit, which makes it difficult to reduce the size of the fuel cell stack.